Transceivers are used in many communication systems including wireless devices, for example. A transceiver can be used for both transmit and receive operations in a device. Often, a transceiver is coupled between a baseband processor and an antenna and related circuitry of the system. In the receive direction, incoming radio frequency (RF) signals are received by the transceiver, which downconverts them to a lower frequency for processing by the baseband processor. In the transmit direction, incoming baseband data is provided to the transceiver, which processes the data and upconverts it to a higher frequency, e.g., a RF frequency. The upconverted RF signals are then passed to a power amplifier (PA) for amplification and transmission via an antenna.
Thus the transceiver acts as an interface between baseband, digital and RF domains. Among the various tasks performed by a transceiver are downconversion and upconversion, modulation and demodulation, and other related tasks. Oftentimes, a transceiver is controlled by a baseband processor to which it is coupled. In turn, the transceiver is coupled to a power amplifier, which generates appropriately conditioned RF signals for transmission via an antenna. In the transmit direction, the transceiver provides RF signals to the power amplifier, which amplifies the signals based on gain and ramp information for a given modulation type. Different wireless communication protocols implement different modulation schemes. For example, a Global System for Mobile communication (GSM) system can implement various modulation schemes, including a gaussian minimum shift keying (GMSK) modulation scheme for voice/data. Furthermore, extensions to GSM, such as Enhanced Data rates for GSM Evolution (EDGE) use other modulation schemes, such as an 8 phase shift keying (8-PSK) modulation scheme.
While many different power amplifiers exist, most PAs receive various control signals, e.g., from the baseband processor, along with the data signal, e.g., from the transceiver. These control signals include an enable signal and the like. Also, a PA receives a supply voltage and one or more bias voltages. Some PAs in certain wireless protocols implement a linear architecture. These PAs can operate in dual modes of operation, namely a saturated mode and a linear mode. GMSK schemes typically transmit in a saturated mode in which transmitted data is output from the PA with a constant amplitude as measured by power versus time during the useful part of the burst. In contrast, 8-PSK modulation implements a linear mode in which a time-varying amplitude is output from the PA as measured by power versus time during the useful part of the burst.
In the 3 GPP specification, a mixed mode of operation is set forth. This mixed mode of operation, otherwise known as a dual transfer mode, switches modulation schemes during inter-slot periods. Specifically, one implementation calls for transmission in alternating GMSK-8-PSK schemes in different slots. Control between the modulation schemes switches in the inter-slot period. Accordingly, the 3 GPP specification requires that a switching spectrum be met in which power transients at the output of the power amplifier are reduced or eliminated in the inter-slot period to provide for proper operation.
In the different modes of operation, leakage currents from a phase-lock loop (PLL) path within a transceiver can affect RF output signals from the transceiver to the power amplifier, particularly in a linear transmitter architecture. Thus in the 8-PSK mode, currents from the PLL or an associated local oscillator, which may be unmodulated carrier signals, should not leak through an upconversion mixer to the output of the transceiver. If this were to occur, offset noise would appear that could violate the output-offset suppression (OOS) requirement of the 3 GPP specification. A need thus exists to prevent such noise from coupling through a mixer.